Method of eliminating boron contamination in annealed wafer

ABSTRACT

A method by which a silicon wafer is prevented from increasing boron concentration near the surface and difference in the boron concentration does not arise between the surface of the annealed wafer and the silicon bulk to eliminate boron contamination in the silicon wafer caused by an annealing treatment is provided. The method includes, when annealing a silicon wafer having a surface on which a native oxide film has formed and boron of environmental origin or from chemical treatment prior to annealing has deposited, steps of carrying out temperature heat-up in a mixed gas atmosphere having a mixing ratio of hydrogen gas to inert gas of 5% to 100% so as to remove the boron-containing native oxide film, followed by annealing in an inert gas atmosphere.

This is the U.S. National Stage of International Application No.PCT/JP03/10934, filed Aug. 28, 2003, which relies for priority onJapanese Patent Application No. 2002-252608, filed Aug. 30, 2002, thecontents of both of which are incorporated herein by reference in theirentireties.

TECHNICAL FIELD

The present invention relates to a method for eliminating boroncontamination in an annealed wafer. The invention relates moreparticularly to a method for eliminating boron contamination in thesurface of a wafer just prior to annealing in order to prevent thediffusion of boron deposits to the interior of the wafer duringhigh-temperature annealing that would cause a decline in themanufacturing yield of devices (semiconductor devices) owing to changesin the electrical characteristics in the wafer surface.

BACKGROUND ART

As the level of device integration rises, crystal defects near thesurface of silicon wafers have a larger impact on the manufacturingyield of semiconductor devices. Therefore, a need exists forhigh-quality silicon wafers that are free of crystal defects. Suchwafers have until now generally been obtained by processes that useepitaxial growth. Wafers known as “annealed wafers” have recently beendeveloped, which are obtained by high-temperature treating (annealing)silicon wafers in an inert gas atmosphere to remove near-surface crystaldefects.

However, there are a number of drawbacks to annealing treatment. Anative oxide film is formed on the wafer surface prior to annealing, andin addition, boron deposits (e.g., BF₃, B₂O₃, and the like) from theenvironment to which the wafer is exposed or from the chemical treatmentfor cleaning the wafer that is carried out prior to annealing are alsopresent. When annealing is carried out in an inert gas atmosphere, theboron deposits diffuse into the wafer interior, increasing thenear-surface boron concentration. As a result, the electricalcharacteristics in the surface near the active region of thesemiconductor device change, lowering the manufacturing yield of thedevices. Because it is extremely difficult to completely prevent thedeposition of boron to the wafer surface during wafer fabrication, therehas existed a desire for a method for eliminating boron contamination;that is, a method which prevents the boron concentration near thesurface of the wafer from increasing as a result of annealing treatment.

To provide a uniform boron concentration near the wafer surface and alsoeliminate crystal defects, Japanese published unexamined application JP2002-100634 discloses a method in which the silicon wafer isheat-treated in a hydrogen gas-containing atmosphere so as to removedeposited boron prior to removal of the native oxide film, then isheat-treated in an inert gas atmosphere. The hydrogen gas concentrationin the atmosphere is preferably from 0.1% to the lower explosion limitof about 4%. However, even with the use of this prior-art method, theboron concentration in the surface of the annealed wafer tends to remainhigher than the boron concentration in a bulk silicon, and so theoutcome has not always been satisfactory.

DISCLOSURE OF THE INVENTION

With the foregoing in view, it is an object of the invention to providea method which eliminates boron contamination associated with theannealing of wafers, and which can thus make the boron concentration inthe wafer surface and the boron concentration in bulk siliconsubstantially the same.

That is, the method for eliminating boron contamination in an annealedwafer of the present invention includes, when annealing a silicon waferhaving a surface on which a native oxide film has formed and boron ofenvironmental origin or from chemical treatment prior to annealing hasdeposited, steps of carrying out temperature heat-up in a mixed gasatmosphere having a mixing ratio of hydrogen gas to inert gas of 5% to100% so as to remove the boron-containing native oxide film, followed byannealing in an inert gas atmosphere.

As noted above, Japanese published unexamined application JP 2002-100634discloses a method which involves carrying out heat treatment in ahydrogen gas-containing atmosphere to remove deposited boron prior toremoval of the native oxide film, followed by heat-treating in an inertgas atmosphere, however, the hydrogen gas concentration in the hydrogengas-containing atmosphere is indicated therein as being preferably from0.1% to the lower explosion limit of about 4%. This range is intended toeliminate the need for a heat-treating furnace having a sealedconstruction that provides greater airtightness and forexplosion-protected equipment, and moreover assumes the use of anormal-pressure furnace. Hence, the possibility of using gas having ahydrogen gas concentration of 5% or more has not been investigatedwhatsoever. Moreover, the temperature range of heat treatment is from900 to 1,100° C., which was presumably arrived at to strike a balancebetween the etching rate of the native oxide film and the vaporizationand diffusion of the deposited boron.

We have conducted investigations on mixed gases including an inert gasand hydrogen gas without limiting the hydrogen gas concentration, and asa result, we have found that by subjecting wafers to temperature heat-upin a mixed gas atmosphere having a hydrogen gas mixing ratio of 5% ormore, followed by annealing treatment in an inert gas atmosphere, aboron concentration in the wafer surface and a boron concentration inthe silicon bulk which are substantially the same can be achieved. At ahydrogen gas mixing ratio of less than 5%, the boron concentration inthe wafer surface becomes much higher than the boron concentration inthe silicon bulk, resulting in a large change in the electricalresistivity. The hydrogen gas mixing ratio is preferably from 10 to 30%.

FIGS. 1A to 1C are schematic views illustrating the steps of removing aboron-containing native oxide film, followed by carrying out annealingtreatment in an inert gas atmosphere according to the method foreliminating boron contamination in an annealed wafer of the presentinvention. FIG. 1A is a schematic view of a cross-section of a polishedwafer that has been polished before being loaded into an annealingfurnace. A native oxide film has formed on the surface, and boron ispresent both on the surface of the oxide film and in the oxide film.FIG. 1B is the stage of heating up after the wafer has been loaded intothe annealing furnace. Since the entire oxide film is removed in a mixedgas atmosphere of argon and hydrogen, the boron in the surface and inthe oxide film are removed. FIG. 1C is the stage in which thetemperature has reached the annealing temperature following thecompletion of temperature heat-up. No oxide film or deposited boron ispresent on the surface of the wafer when it is treated in an inert gasatmosphere.

In the present invention, a treatment temperature of the temperatureheat-up in the mixed gas atmosphere can be set from 700 to 1,200° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, and 1C are schematic views of steps in the method foreliminating boron contamination in an annealed wafer of the presentinvention.

FIG. 2 is an example of a process chart for carrying out the method foreliminating boron contamination in an annealed wafer of the presentinvention.

FIG. 3 shows the boron concentration and resistivity profiles in thedepth direction of the wafer, as measured by the spreading resistance(SR) method in Example 1.

FIG. 4 shows the boron concentration and resistivity profiles in thedepth direction of the wafer, as measured by the SR method in Example 2.

FIG. 5 shows the boron concentration and resistivity profiles in thedepth direction of the wafer, as measured by the SR method in Example 3.

FIG. 6 shows the boron concentration and resistivity profiles in thedepth direction of the wafer, as measured by the SR method in Example 4.

FIG. 7 shows the boron concentration and resistivity profiles in thedepth direction of the wafer, as measured by the SR method inComparative Example 1.

FIG. 8 is a graph of the ratio of the wafer surface boron concentration(C_(S)) to the boron concentration in bulk (C_(B)) versus the hydrogengas mixing ratio.

FIG. 9 is a graph in which the wafer surface boron concentration (C_(S))and the boron concentration in bulk (C_(B)) are each plotted separatelyagainst the hydrogen gas mixing ratio.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described below. FIG. 2 is anexample of a process chart showing parameters such as temperatures andgas compositions with time in a temperature heat-up process, anannealing treatment process, and associated processes therewith.

The processes shown in FIG. 2 are explained. Here, argon gas is used asan inert gas. First, a wafer is loaded into an annealing furnace at 700°C. and is purged using argon gas only. Next, the temperature is heatedup to 1,200° C. at a rate of 5° C./min in a mixed gas atmosphere ofargon and hydrogen having a given ratio. The atmosphere is then changedto argon gas only, and annealing is carried out at 1,200° C. for 1 hour.After the temperature is subsequently lowered to 700° C. at −3° C./min,the wafer is removed from the annealing furnace.

The present invention will now be described specifically with examples,however, it is to be understood that the present invention is notlimited to these examples.

EXAMPLE 1

A polished wafer (PW) of P-type silicon having a (100) plane orientationwhich was obtained by slicing from an ingot grown by the Czochralskitechnique and polishing, and which had an electrical resistivity of 20Ω·cm, an oxygen concentration of 1E18 atom/cm³, and a diameter of 200mm, was used. The polished wafer was washed twice with SC-1 (a mixtureof ammonia, hydrogen peroxide, and water), and then washed withhydrochloric acid, followed by been loaded into an annealing furnace.This wafer was held in a clean room for about one week, and with anative oxide film formed on the wafer surface and boron deposits alsopresent on the surface, the wafer was then furnished for testing.Annealing was carried out as shown in FIG. 2, and in a temperatureheat-up process, a mixed gas having a mixing ratio of hydrogen gas toargon gas of 5% was used.

The resulting annealed wafer was measured by a spreading resistance (SR)method to determine changes in boron concentration and electricalresistivity in a depth direction near the surface of the wafer. Thechange in boron concentration in the depth direction near the surface ofthe wafer was also measured by a secondary ion mass spectroscopy (SIMS).

EXAMPLE 2

Except for using a mixed gas having a mixing ratio of hydrogen gas toargon gas of 25% during heating up, annealing was carried out in thesame way as in Example 1, and the resulting annealed wafer was similarlymeasured.

EXAMPLE 3

Except for using a mixed gas having a mixing ratio of hydrogen gas toargon gas of 50% during heating up, annealing was carried out in thesame way as in Example 1, and the resulting annealed wafer was similarlymeasured.

EXAMPLE 4

Except for using 100% hydrogen gas during heating up, annealing wascarried out in the same way as in Example 1, and the resulting annealedwafer was similarly measured.

COMPARATIVE EXAMPLE 1

Except for using a mixed gas having a mixing ratio of hydrogen gas toargon gas of 1% during heating up, annealing was carried out in the sameway as in Example 1, and the resulting annealed wafer was similarlymeasured.

FIGS. 3 to 7 show boron concentration profiles and electricalresistivity profiles in the wafer depth direction, as determined by theSR method, for Examples 1 to 4 and Comparative Example 1. FIG. 8 shows agraph of a ratio (C_(S)/C_(B)) of a surface boron concentration (C_(S))to a boron concentration in a bulk silicon (C_(B) ) versus the hydrogengas concentration. FIG. 9 is a graph in which C_(S) and C_(B) are eachplotted separately against the hydrogen gas mixing ratio.

It is apparent from these results that at a hydrogen gas mixing ratio of5% or more in the mixing gas during heating up, there is littledifference between C_(S) and C_(B). The same can be said concerning theresistivity as well. Moreover, there is substantially no difference inFIG. 9 between C_(S) and C_(B) when the hydrogen gas mixing ratio in themixed gas is from 10 to 30%, indicating this to be an even morepreferable range in the hydrogen gas mixing ratio. On the other hand, ata hydrogen gas mixing ratio of less than 5%, C_(S) is much larger thanC_(B), greatly changing the electrical resistivity. It can be concludedfrom these results that to achieve the object of the invention thehydrogen gas mixing ratio in the mixed gas during heating up must be 5%or more, and is preferably 10 to 30%.

The boron concentration profiles in the wafer depth direction determinedby SIMS in Examples 1 to 4 and Comparative Example 1 showed the sametrend as the foregoing boron concentration profile results obtained bythe SR method. The samples obtained at a hydrogen gas mixing ratio of25% in the mixed gas had the flattest boron concentration profile, andwere thus found to be desirable. At a hydrogen gas mixing ratio of 50%or more, the surface boron concentration tended to decrease, whichagreed with the above results indicating the preferred hydrogen gasmixing ratio in the mixed gas to be from 10 to 30%.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, when annealing asilicon wafer having a surface on which a native oxide film has formedand on which boron of environmental origin or from chemical treatmentprior to annealing has deposited, by heating up the wafer in a mixed gasatmosphere having a mixing ratio of hydrogen gas to an inert gas of 5%to 100% so as to remove the boron-containing native oxide film, followedby annealing the wafer in an inert gas atmosphere, difference in boronconcentration does not arise between the surface of the annealed waferand the silicon bulk. As a result, the electrical characteristics in thewafer surface do not change, making it possible to reliably andeffectively prevent a decline in the manufacturing yield ofsemiconductor devices.

1. A method for eliminating boron contamination in an annealed wafer,the method comprising, annealing a silicon wafer having a surface onwhich a native oxide film has formed containing boron of environmentalorigin or from chemical treatment prior to the annealing, wherein in theannealing, the silicon wafer is subjected to temperature heat-up in amixed gas atmosphere having a ratio of hydrogen gas to an inert gas of5% to 100% so as to remove the boron-containing native oxide film, andsubsequent annealing in an inert gas atmosphere, and a boronconcentration in the wafer surface and a boron concentration in the bulksilicon are made to be substantially same by the annealing, wherein aratio of the boron concentration in the wafer surface to the boronconcentration in the bulk silicon is controlled to be 0.78 to 1.10. 2.The method for eliminating boron contamination in an annealed waferaccording to claim 1, wherein a treatment temperature of the temperatureheat-up in the mixed gas atmosphere is from 700° C. to 1,200° C.
 3. Themethod for eliminating baron contamination in an annealed wateraccording to claim 1, wherein the temperature heat-up is carried out inthe mixed gas atmosphere in which the ratio of the hydrogen gas to theinert gas is from 10% to 30%.
 4. The method for eliminating boroncontamination in an annealed wafer according to claim 2, wherein thetemperature heat-up is carried out in the mixed gas atmosphere in whichthe ratio of the hydrogen gas to the inert gas is from 10% to 30%.